1. Field of the Invention
This invention relates to the production of ozone for the sterilization of point of use potable water sources such as reverse osmosis systems, refrigerators, drinking fountains, etc.
2. Background of the Related Art
Ozone has long been recognized as a useful chemical commodity valued particularly for its outstanding oxidative activity. Because of this activity, it finds wide application in disinfection processes. In fact, it kills bacteria more rapidly than chlorine, it decomposes organic molecules, and removes coloration in aqueous systems. Ozonation removes cyanides, phenols, iron, manganese, and detergents. It controls slime formation in aqueous systems, yet maintains a high oxygen content in the system. Unlike chlorination, which may leave undesirable chlorinated organic residues in organic containing systems, ozonation leaves fewer potentially harmful residues. Ozone has also been shown to be useful in both gas and aqueous phase oxidation reactions that may be carried out by advanced oxidation processes (AOPs) in which the formation of hydroxyl radicals (OH•) is enhanced by exposure to ultraviolet light. Certain AOPs may even involve a catalyst surface, such as a porous titanium dioxide photocatalyst, that further enhances the oxidation reaction. There is even evidence that ozone will destroy viruses. Consequently, it is used for sterilization in the brewing industry and for odor control in sewage treatment and manufacturing. Ozone may also be employed as a raw material in the manufacture of certain organic compounds, e.g., oleic acid and peroxyacetic acid.
Thus, ozone has widespread application in many diverse activities, and its use would undoubtedly expand if its cost of production could be reduced. For many reasons, ozone is generally manufactured on the site where it is used. However, the cost of ozone generating equipment and poor energy efficiency of producing ozone has deterred its use in many applications and in many locations.
On a commercial basis, ozone is currently produced by the silent electric discharge process, otherwise known as corona discharge, wherein air or oxygen is passed through an intense, high frequency alternating current electric field. The corona discharge process forms ozone (O3) through the following reaction:3/2O2=>O3; ΔH°298=34.1 kcal
Yields in the corona discharge process generally are in the vicinity of 2% ozone, i.e., the exit gas may be about 2% O3 by weight. Such O3 concentrations, while quite poor in an absolute sense, are still sufficiently high to furnish usable quantities of O3 for the indicated commercial purposes. Another disadvantage of the corona process is the production of harmful nitrogen oxides (NOx). Other than the aforementioned electric discharge process, there is no other commercially exploited process for producing large quantities of O3.
However, O3 may also be produced through an electrolytic process by impressing an electric current (normally direct current (DC)) across electrodes immersed in an electrolyte, i.e., electrically conducting fluid. The electrolyte includes water which is dissociated by the electrolytic process into its respective elemental species, O2 and H2. Under the proper conditions, the oxygen is also evolved as the O3 species. The evolution of O3 may be represented as:3H2O=>O3+3H2; ΔH°298=207.5 kcal
It will be noted that the ΔH° in the electrolytic process is many times greater than that for the electric discharge process. Thus, the electrolytic process appears to be at about a six-fold disadvantage.
More specifically, to compete on an energy cost basis with the electric discharge method, an electrolytic process must yield at least a six-fold increase in ozone yield. Heretofore, the necessary high yields have not been realized in any foreseeable practical electrolytic system.
The evolution of O3 by electrolysis of various electrolytes has been known for well over 100 years. High yields up to 35% current efficiency have been noted in the literature. Current efficiency is a measure of ozone production relative to oxygen production for given inputs of electrical current, i.e., 35% current efficiency means that under the conditions stated, the O2/O3 gases evolved at the anode are comprised of 35% O3 by weight. However, such yields could only be achieved utilizing very low electrolyte temperatures, e.g., in the range from about −30° C. to about −65° C. Maintaining the necessary low temperatures, obviously requires costly refrigeration equipment as well as the attendant additional energy cost of operation.
Ozone is present in large quantities in the upper atmosphere of the earth to protect the earth from the suns harmful ultraviolet rays. In addition, ozone has been used in various chemical processes and is known to be a strong oxidant, having an oxidation potential of 2.07 volts. This potential makes it the fourth strongest oxidizing chemical known.
Because ozone has such a strong oxidation potential, it has a very short half-life. For example, ozone which has been solubilized in waste water may decompose in a matter of 20 minutes. Ozone can decompose into secondary oxidants such as highly reactive hydroxyl radicals (OH•) and peroxyl radicals (HO2•). These radicals are among the most reactive oxidizing species known. They undergo fast, non-selective, free radical reactions with dissolved compounds. Hydroxyl radicals have an oxidation potential of 2.8 volts (V), which is higher than most chemical oxidizing species including O3. Most of the OH• radicals are produced in chain reactions where OH• itself or HO2 • act as initiators.
Hydroxyl radicals act on organic contaminants either by hydrogen abstraction or by hydrogen addition to a double bond, the resulting radicals disproportionate or combine with each other forming many types of intermediates which react further to produce peroxides, aldehydes and hydrogen peroxide.
Electrochemical cells in which a chemical reaction is forced by added electrical energy are called electrolytic cells. Central to the operation of any cell is the occurrence of oxidation and reduction reactions which produce or consume electrons. These reactions take place at electrode/solution interfaces, where the electrodes must be good electronic conductors. In operation, a cell is connected to an external load or to an external voltage source, and electric charge is transferred by electrons between the anode and the cathode through the external circuit. To complete the electric circuit through the cell, an additional mechanism must exist for internal charge transfer. This is provided by one or more electrolytes, which support charge transfer by ionic conduction. Electrolytes must be poor electronic conductors to prevent internal short circuiting of the cell.
The simplest electrochemical cell consists of at least two electrodes and one or more electrolytes. The electrode at which the electron producing oxidation reaction occurs is the anode. The electrode at which an electron consuming reduction reaction occurs is called the cathode. The direction of the electron flow in the external circuit is always from anode to cathode.
Recent ozone research has been focused primarily on methods of using ozone, as discussed above, or methods of increasing the efficiency of ozone generation. For example, research in the electrochemical production of ozone has resulted in improved catalysts, membrane and electrode assemblies, flowfields and bipolar plates and the like. These efforts have been instrumental in making the electrochemical production of ozone a reliable and economical technology that is ready to be taken out of the laboratory and placed into commercial applications.
However, because ozone has a very short life in the gaseous form, and an even shorter life when dissolved in water, it is preferably generated in close proximity to where the ozone will be consumed. Traditionally, ozone is generated at a rate that is substantially equal to the rate of consumption since conventional generation systems do not lend themselves to ozone storage. Ozone may be stored as a compressed gas, but when generated using corona systems the output gas stream is essentially at atmospheric pressure. Therefore, additional hardware for compression of the gas is required, which in itself reduces the ozone concentration through thermal and mechanical degradation. Ozone produced by the corona process may also be dissolved in liquids such as water but this process generally requires additional equipment for introducing the ozone gas into the liquid, and at atmospheric pressure and ambient temperature only a small amount of ozone may be dissolved in water.
Because so many of the present applications have the need for relatively small amounts of ozone, it is generally not cost effective to use conventional ozone generation systems such as corona discharge. Furthermore, since many applications require the ozone to be delivered under pressure or dissolved in water, as for disinfecting, sterilizing, treating contaminants, etc., the additional support equipment required to compress and/or dissolve the ozone into the water stream further increases system cost.
Therefore, there is a need for an ozone generator system that operates efficiently on standard AC or DC electricity and water to deliver a reliable stream of ozone gas that is generated under pressure for direct use by the application. Still other applications would benefit from a stream of highly concentrated ozone that is already dissolved in water where it may be used directly or diluted into a process stream so that a target ozone concentration may be achieved. It would be desirable if the system was self-contained, self-controlled and required very little maintenance. It would be further desirable if the system had a minimum number of wearing components, a minimal control system, and be compatible with low voltage power sources such as solar cell arrays, vehicle electrical systems, or battery power.